Gray.

Gray is a sky, filled with gloom.

It's an ageless mountain.

It's a dusty book, filled with ancient secrets.

It has the wisdom of an old man.

Gray is knowledge.

It's the storm clouds before the rain.

It's gravel on a cold road.

It's a rocky cave.

But it's also a grand castle.

Gray is sorrow.

It's the smoke rising from a hot flame.

It's as crafty as a fox.

It's a shadow in the corner.

It's the horizon in the very early morning.

It's also the ashes after a fire.

Gray is never predictable.

As you can see, gray is both wisdom and greif.